Coral probiotics promote tissue-specific and putatively beneficial microbiome restructuring in a coral dwelling fish

The ongoing fourth mass global coral bleaching event reinforces the need for active solutions to
support corals through this major crisis. The use of beneficial microorganisms for corals (BMCs),
offers a promising nature-based solution to rehabilitate coral’s dysbiotic microbiomes. While
the benefits to corals are increasingly recognized, the impacts on associated reef organisms,
such as fish, remain unexplored. This study investigated the effects of BMCs on the tissue-
associated microbiomes of Dascyllus abudafur (Pomacentridae), a damselfish that lives closely
associated with coral colonies. Over three months, we applied BMCs three times per week to
healthy Pocillopora verrucosa colonies in the central Red Sea and analyzed the resultant
changes in the inhabiting fish's microbiomes. Our findings reveal significant, tissue-specific
shifts in bacterial communities, particularly in skin and gut, with moderate changes observed in
gills. Notably, putative fish beneficial bacteria such as Mitsuokella spp. were enriched in the
skin, while various Firmicutes taxa increased in the gut. There was also a marked decrease in
potential fish pathogens. This study not only highlights the potential extended benefits of BMCs
on coral reef fish but also sets a foundation for understanding the broader ecological
interactions between BMCs and reef-associated organisms.